Polyurethane foam is prepared commercially in the form of large blocks that are subsequently cut into the desired shape for use in the manufacture of various articles that require a foam padding. It is well known that polyurethane foam requires time to cure and develop its full physical properties. In typical polyurethane foam-forming formulations, a polyhydroxy material ("polyol"), water and an organic isocyanate compound are reacted in the presence of catalysts or other additives. Much of the time, a small percentage of terminal isocyanate groups are left unreacted in the foam structure. If the foam is distorted or compressed in this condition, it fails to recover its original dimensions when the distortive or compressive force is released. Normally, the terminal isocyanate groups that are left unreacted in the foam structure will react with the residual water in the foam structure or with the water vapor in the atmosphere over a period of several hours or days, and the foam will ultimately achieve its full physical properties.
Post-curing of polyether derived foam by exposure to a mixture of water vapor and gaseous ammonia, primary or secondary amines at temperatures of about 50.degree. to 150.degree. F. for a period of at least one minute is disclosed in U.S. Pat. No. 4,537,912. While this process effectively and rapidly cures the foam, it constitutes an additional step beyond those normally used in the foam manufacturing process. This process also requires storage of the foam prior to the post-curing treatment. It is more desirable to cure the foam as it is manufactured to reduce or even eliminate such storage time and to prepare a fully cured foam material which can be immediately shipped to the end users after cutting to the desired shapes.
U.S. Pat. No. 4,757,094 discloses that melamine powder can be added to the foam-forming components of a flexible polyether derived polyurethane foam formulation to cure the foam during or immediately after the foam forming step. Also, U.S. Pat. No. 4,757,093 discloses that melamine powder can be substituted for a portion of the liquid phosphorus flame retardant ester in such foam-forming formulations without loss of flame retardance in the resultant foam.
U.S. Pat. No. 3,726,835 discloses that melamine or dicyandiamide can be utilized as a stabilizer for polyurethane prepolymers which are thereafter cured to form elastomeric polymers. In these compositions, 10 parts melamine or dicyandiamide is added to 100 parts prepolymer.
U.S. Pat. No. 3,824,239 discloses triazine compositions which can be used as catalysts for the preparation of rigid cellular foams.
U.S. Pat. No. 4,143,029 discloses dripless, fireretarded, polyurethane elastomers which include certain melamine derivatives therein.
U.S. Pat. No. 4,374,207 discloses flexible, resilient, polyurethane foam having improved flame retardancy and intumescent properties prepared from a reaction mixture comprising a polyether polyol, an organic polyisocyanate, a blowing agent, a surfactant, a catalyst, a flame retardant and hydrated alumina, optionally with a char former of a melamine derivative.
U.S. Pat. Nos. 4,139,501 and 4,197,373 disclose polyether polyurethane foams containing, as a flame retardant additive, a melamine derivative, usually in an amount ranging from one to 20 weight percent of the polyol (in the '501 patent) and from 0.25 to 30 parts by weight based on 100 parts polyol in the '373 patent. The '501 patent also utilizes conventional halogenated esters in amounts ranging from 4 to 30 percent by weight of the polyol to further increase the flame retardance.
U.S. Pat. Nos. 4,708,984, 4,710,542 and 4,742,118 relate to cross-linking agents of aminotriazoles, aminotriazines, and S-triazines, respectively, for use in coatings.
U.S. Pat. No. 4,699,933 relates to a polyurethane composition which contains triazine groups prepared by reacting certain oligomers.
U.S. Pat. No. 4,722,942 relates to active hydrogen-containing compositions suitable for the production of flexible polyurethane foam when reacted with a polyisocyanate. These compositions contain, in relevant part, polycarbamates as one of a group of suitable polyfunctional flame retardant additives.
None of these references disclose the possibility of using a curing agent of a melamine derivative for rapidly curing flexible polyether polyurethane foam while concommittantly improving the compression set properties during foam formation, nor do they disclose the benefits of substituting small amounts of such curing agents for a portion of the conventional liquid phosphorous esters to maintain the desired flame retardance of the foam.
The present invention provides a one-step foaming and curing process which achieves a rapid and full post cure of the foam so that low compression set values as measured by ASTM standard test D-3574 (Constant Deflection Compression Set Test) are obtained, with the elimination of post curing steps, and with a reduction of conventional liquid phosphorus flame retardant esters while retaining the same degree of flame retardance of the foam.